Magnesia stabilized zirconia fired ceramics are known in the art (see U.S. Pat. No. 3,365,317, for example). Magensia combinations with other stabilizers such as Y.sub.2 O.sub.3, CaO, CeO.sub.2 and Cr.sub.2 O.sub.3 are also known in the art as available stabilizers for zirconia (see U.S. Pat. No. 2,040,215). Unfortunately however, magnesia-stabilized zirconia in the cubic structure form only remains so stabilized providing it is not subjected to long exposures (e.g., 50-60 hours) of temperatures within the range of 1,650.degree. F. and 2,250.degree. F. Within this temperature destabilization occurs, occasioned by the precipitation of periclase from the cubic ZrO.sub.2 solid solution. As the periclase precipitates in that temperature range, the ZrO.sub.2 cubic crystallites transform into the tetragonal structure then to the monoclinic form upon cooling with consequent volume changes and internal stresses which weaken the body. The destabilization is discussed in Ryshkewitch, Oxide Ceramics, Academic Press, (1964) page 358, et seq.
The destabilization of magnesia-stabilized zirconia may occur if the zirconia body is held for a period of time in the temperature range 1,650.degree. to 2,550.degree. F. What is more important, however, is that the destabilization may occur to the zirconia body during a use in which the body is exposed on many occasions for a relatively short time (maybe 2-3 hours) on each occasion, i.e., cycled through the 1,650.degree.-2,550.degree. F. range. This temperature cycling is typical of treatment afforded die nibs, setter plates, crucibles, metal pouring nozzles, heat exchangers, and grinding beads, among many other products formed of stabilized zirconia. The present invention is useful in products such as those mentioned.
The temperature cycle which the above products undergo in actual use is, in reality, room temperature up to the use temperature and back to room temperature. For a zirconia body, the more narrow range of 1,650.degree.-2,550.degree. F. is the most dangerous with respect to thermal stress and ultimately mechanical strength because the crystal structure changes on cooling in this range from tetragonal to monoclinic with a volume expansion of about 9%. To reinforce this instability range, the discussion herein may reiterate the 1,650.degree.-2,550.degree. F. temperatures, although the actual thermal cycling encompasses the larger range of 400.degree. to 2,500.degree. F.
The other cause of thermal stress in the zirconia body at all temperatures is the stress caused by rapid temperature changes which result in thermal gradients within the body. The magnitude of these stresses in the body and their effect on the strength are determined in part by the coefficient of thermal expansion of the material. These two causes of internal stress (structural transformation and thermal expansion) generally require narrow limits on the amount of stabilizer in the batch. Primarily, the stabilizer is a desirable addition which brings about stabilization of ZrO.sub.2 into the cubic or tetragonal structure. Secondarily, the stabilizer also brings about an increase in the thermal expansion coefficient of the composition. With these considerations (and where the thermal expansion of the zirconia body must be kept low), the addition of any amount of stabilizer in excess of that required to bring about full stabilization would be detrimental to the ability of the body to withstand cyclical and rapid exposures to varying temperatures.
Experience in the art has shown that magnesia-stabilized zirconia bodies may be matured (viz., develop ceramic bonding and become impervious as observed by dye penetration) at greater than about 2.5% MgO when fired to within a wide range of temperatures. Unfortunately, the destabilization which was described above characterizes the magnesia-zirconia compositions.
Yttria, on the other hand, is not known to precipitate rapidly from zirconia solid solution, and therefore a zirconia body stabilized with yttria remains stable during thermal cycling over long periods of time. Yttria, however, must be present in quantities greater than 4% in order to produce a mature, partially stabilized zirconia refractory and greater than about 12% to produce a fully cubic body. The firing temperature should also be in excess of 3,000.degree. F. to mature the body. Before the present invention, bodies containing less than 4% Y.sub.2 O.sub.3 were not stable or impervious to dye penetration and not useful for most commercial applications because of low strength, high porosity and general immaturity.
In U.S. Pat. No. 2,427,034 (Wainer), zinc and other group II oxides were suggested as additives to MgO stabilized zirconias, but therein Wainer thought it necessary to have a ZrO.sub.2 :MgO mole ratio of between 1/2 and 2 for satisfactory fired bodies (i.e., at least about 14% by weight MgO), and in any case a greater molal percentage of magnesia than group II oxide in the body.
In U.S. Pat. No. 3,410,728, Fullman, et al., disclose the use of ZnO in stabilized zirconia to effect electric conductivity in a fuel cell. The patentees, however, define the stabilized zirconia as a compound with a cubic crystal structure and are therefore disclosing only conventional cubic stabilized bodies using relatively high levels of stabilizers.